A number of London Tube stations were evacuated and some lines closed after minor blasts occurred in what the Metropolitan Police described as a "serious incident" on Thursday.
BBC News correspondents reported on a day that London came under attack for a second time.
Margaret Gilmore : Shepherd's Bush : 2210
The police can tell an awful lot from the eyewitness accounts from those on the train.
They saw white smoke and could smell rubber. It seems likely that the detonators went off, but the devices did not explode.
One theory the police are investigating is that if these devices were from the batch of bombs used two weeks ago, they may have deteriorated over time and that is one reason why they may have not gone off properly.
The police know what a prize it would be if they caught the people responsible.
If they are connected to the suicide bombings of two weeks ago, the police could begin to unravel the whole plot.
Bridget Kendall : Oval Tube station : 2200
Exactly what happened here still isn't clear.
One unconfirmed report - sourced to the London Transport Police - suggests the suspect may have tried to use a handgun to fire at the back-pack in an attempt to detonate it.
Mike Wooldridge : Shoreditch : 1915
The cordon around the bus is not only still here, in the last couple of hours it has been moved further out.
When the police first checked out the bus we could get within 50 yards of it, but as the day has progressed more people have been moved out of the way and more offices in the area have been evacuated.
Bridget Kendall : Oval Tube station : 1820
I'm standing less that 100 yards from the Oval Tube station. Whatever the police have found down there, they are not telling us.
Life is starting to return to normal.
Schoolchildren in their uniforms are walking home from school looking curiously at what is going on and cars are going past. There is no sense of panic.
Duncan Kennedy : Shepherd's Bush : 1810 BST
The police are keeping the information flow very much to themselves.
Quite a large area around the Tube station has been closed off and the police are crawling all over it, but they are not saying where their efforts are precisely focussed.
We are not sure if it is on the train, in the station, or on the concourse.
In the last five minutes some of the main roads have been reopened so, as far as everyday life goes, things are starting to get back to normal.
Annita McVeigh : Shoreditch : 1800 BST
People don't know what to make of what is going on here.
The emergency services are carefully looking at what has happened and there is a lot of activity here.
Fire engines and almost a dozen police cars are in the area, but at the moment the atmosphere is calm but rather tense.
Brian Wheeler : Oval Tube station : 1755 BST
The streets around Oval station remain sealed off for an area of about a square mile.
Residents attempting to return home after work have been advised by police to stay elsewhere tonight.
Mary Hammett, 46, said: "The Underground runs under our flats so maybe that's why.
"I am OK, I have friends I can stay with. The emergency services have been brilliant and I think people's attitude has been brilliant as well."
Gill Higgins : University College Hospital : 1715 BST
There is still a cordon up behind us. People cannot enter the hospital but they can now leave it.
Some people have described quite a sense of panic as armed police officers were rushing around searching the building, with doors locked so nobody could leave at that time.
We've had an email handed to us saying police asked staff to be on the lookout for a man on their behalf.
It said people should look for a black or possibly Asian man, of about six feet two inches tall, wearing a blue top with a hole, and possibly having wires protruding from the shirt. People were told not to approach this man if they saw him but to dial 999.
We've heard that the incident here is concluded but the cordons are still up and there is still a police presence inside.
Annita McVeigh : Shoreditch : 1705 BST
I'm on Shoreditch High Street. The bus is further back round the corner near Columbia Road, where there is a busy flower market every Sunday.
For the second time in two weeks a bus has been at the centre of a sinister incident.
The passengers abandoned the bus as soon as they heard the explosion. One passenger said he thought it sounded something like a large stone hitting the side of the bus.
Many will have had memories of Tavistock Square fresh in their minds.
There is a lot of activity here. In front of me there are about a dozen police officers ready to respond, but at the moment things are calm.
I think people are quite bewildered and loathe to rush to a judgement.
Duncan Walker : Shepherd's Bush : 1645 BST
Already shaken by the bombings a fortnight ago, the latest alert has only heightened the anxiety felt by some.
Dental nurse Louise Graham, 21, said she would be walking all the way home to Putney, several miles away. "Luckily all my friends live around where I am, otherwise I will drive everywhere."
Sarah Rafique, who was shopping with her sister Tahira said: "It really worries you, when I step onto a Tube or a bus now, I think twice."
Daniel Sandford : BBC Television Centre : 1630 BST
The undercurrent in these police statements is that these might have been much more serious incidents, that these might have been serious bombs which might not have gone off properly.
The concern is that this could have turned out to be another serious attack on London.
There is now a forensic investigation underway, and also a massive manhunt.
Gill Farrington : Shepherd's Bush : 1630 BST
I heard three or four muffled bangs coming from the Tube station.
They happened about five minutes apart. The whole area has been evacuated. All the shops are closed.
When some people refused to move, they were pushing them along saying: "You have to move for your own safety."
Ollie Stone-Lee : Warren Street station : 1620 BST
A group of Hare Krishna worshippers have approached the edge of the cordon playing drums and singing. It is not clear if they were passing or came because they heard what was happening.
A police officer has discreetly told them that, with many families upset by today's incident, who might not know much about Hare Krishna, singing might be seen as inappropriate and has said would they mind going elsewhere.
One of the worshippers said they had come to the cordon specifically.
He said: "We are very sorry about what has happened, we are part of London culture." He said they had come to pray.
Before that Stephan Scheuss, 26, who is from Switzerland but temporarily working in London, said: "It is scary, I think I will try to avoid taking the Tube."
Asked if latest incidents would make him return sooner, Mr Scheuss said he planned to go back next weekend anyway.
Janet Barrie : New Scotland Yard : 1610 BST
We've just heard in the last couple of minutes from Sir Ian Blair, the Metropolitan Police Commissioner.
He said there were four incidents.
There was some kind of explosion at Warren Street and Oval stations, either on the platforms or the Tubes.
And there was definitely some kind of explosion on the Number 26 bus in Hackney, as well as an attempt to set off a device on the Hammersmith and City Line.
He said these were very much smaller devices than those seen two weeks ago. He said some had failed to go off or hadn't gone off properly.
And he said the police were in the process of gathering forensic material.
He also said this was a very worrying development, but that this could be a potential breakthrough, as the forensic and other evidence they are gathering now could help them with other enquiries.
The message going out to Londoners tonight is it's time to get back on the Tubes and buses and to get London moving again.
Ollie Stone-Lee : Warren Street station : 1557 BST
Three police cars, one unmarked, have gone into the cordoned-off areas, other emergency vehicles are now leaving the scene - one fire engine and one police mobile response unit.
Duncan Walker : Shepherd's Bush : 1551 BST
Details of just what has happened at Shepherd's Bush remain unclear, with onlookers trading the few scraps of information they have.
Chief Inspector John Sutherland told reporters that the evacuation of the area was completed, with a 400-metre cordon.
He would not confirm whether or not there had been an explosion, and police hoped to reopen the area as soon as possible.
Anna Holford : Oval Tube station : 1543 BST
One of the police officers earlier did tell me that as far as she understood it there was bang here at Oval, smoke and foam were seen coming from a bag and then two Asian men were seen running from the scene.
This police officer also did tell me that one of those men was detained at Embankment station, one of the Tube stations not too far from here.
I must also stress that this is just the report from one police officer, not from the central command here.
Reeta Chakrabarti : Whitehall : 1538 BST
The scene in Westminster had been quite normal here, then all of a sudden there was a huge flurry of police activity.
A number of police cars sped up Whitehall and an armed policeman ran up the road.
The next thing we saw was a man dressed in dark trousers and a dark shirt who was forced to the floor. He was outside the Ministry of Defence building opposite Downing Street.
He was kept on the ground for two or three minutes. And then he stood up very slowly and was handcuffed and then was led away, apparently to another government building.
He didn't appear to be carrying anything.
Ollie Stone-Lee : Warren Street station : 1530 BST
Police have now allowed traffic access through Torrington Place and into Tottenham Court Road. The cordon around Warren Street Tube station is still in existence, but has now been reduced to 200 metres from the station.
The media were allowed in just minutes earlier, leading those present to believe that police did not think there was any massive threat from an explosive device.
Police officers say the Northern Line is expected to be closed for some time.
Businesses along Tottenham Court Road - approximately 100 - have been evacuated. Access to Warren Street Tube station itself is still blocked.
Ellie Reuter : Trafalgar Square : 1527 BST
I'm standing on the corner of the square looking down Whitehall, which is totally cordoned off by the police.
There are a significant number of police cars here.
The police have only just finished clearing the pubs and restaurants down Whitehall and many people here are getting irritated that they can't get to their houses.
I spoke to the police a moment ago and all they could tell me was that there had been an "incident".
Brian Wheeler : Oval Tube station : 1525 BST
I've just spoken to a man who said he was in the ticket hall when he heard screaming and an argument coming from the platform.
A man then came running out, being chased. As he ran he was shouting "what's wrong with these people?".
The eyewitness said the man, who ran out of the station, was panicking. The man I spoke to stressed that he heard no explosion.
Clark Ainsworth : University College Hospital : 1515 BST
Apart from the echoing sounds of sirens since the Warren Street Tube incident, the area does seem to be oddly calm.
Police are slowly moving back the cordons as office workers and crowds of journalists mull the latest rumours.
Ahmed Laarbi, 29, of Stamford Hill, north London, was the focus of world cameras when police questioned and frisked him about his journey plans and behaviour.
He said he didn't mind being questioned and felt the police were doing their job, but said everyone should have been questioned, not just him.
"I am a Muslim; I live in this country, this is my country too. I am working for my family for their future," he told me.
Robert Allen : Putney : 1513 BST
A police officer has told us they are investigating a possible suspect package on a Number 37 bus from Putney on its way to Peckham. The bus is off Putney Hill in St John's Avenue.
There is a police cordon around the area and traffic is at a standstill.
Ollie Stone-Lee : Warren Street : 1512 BST
Crowds are huddled round Tottenham Court Road's famous electrical shops gazing at widescreen television screens, anxious for news.
It is the same scene at a nearby street florist where people bend their ears to the radio trying to catch what has happened.
One woman puts her hand to her mouth in horror as a policeman explains what is happening. "We don't know yet...," he tells her.
At the police cordon Sofiane Mohellebi is recalling the panic which ended his first trip on the Tube since the 7 July bombings.
In his hands are the ladies' shoes which he picked up as people ran from his carriage, some panicking, at the smell of smoke.
He saw no one injured, saw no smoke and heard no explosion, but he said the wait in the tunnel for the train to get into nearby Warren Street station was "agonising".
Crowds gathered at the police cordon, members of the public mingling in the media scrum trying to hear the first-hand account of the Tube passengers.
Most of the crowds have gradually moved away and the police have now extended the cordon down the road.
One police officer said it was normal procedure for the cordon to be set 400 metres from the incident scene.
Police cars and motorbikes with their sirens blazing continue to go through to the scene intermittently.
Duncan Walker : Shepherd's Bush : 1510 BST
The usually bustling Shepherd's Bush market, through which the Hammersmith and City Tube line runs, has been closed because of the alert.
The move has caused concern.
"It was only after an hour that the police told us to shut the market," says one trader, who suggested officers were slow to act.
Another stall holder is upset that this has happened in such a multicultural area.
"It's scaring innocent people who have done nothing. This is a mixed market, with everything from the English to the Irish, to Africans and Muslims."
Duncan Kennedy : Shepherd's Bush : 1505 BST
There is a lot of police activity - police cars rushing to this scene but no ambulances, which does give one the impression that there have not been many serious casualties here.
For those who don't know this area, Shepherd's Bush is a major traffic thoroughfare. Many roads coming in to this area are now sealed off.
Dozens of officers, many, many vehicles - all sorts of disciplines are being brought in here - we've seen a dog unit, someone thought they saw earlier an explosions unit.
Gordon Corera : BBC Television Centre : 1457 BST
On first sight there are similarities to what happened two weeks ago.
But the timing is slightly odd, much later in the day rather than rush hour for maximum damage.
Secondly on the professionalism side, it looks like these explosions didn't go off. It seems possible that the detonator went off but not the main charge.
The question that will be asked is, is this a copycat or a follow-on attack?
Either way, there are a lot of leads for the police to pursue.
Alison Freeman : Warren Street : 1455 BST
Firefighters dressed in bright orange chemical suits entered Warren Street station.
They emerged later, some have removed them, so they can be hosed down. It is not clear whether this is a precaution.
Police are repeatedly making the cordon around the station larger and have warned people to stay inside for their own safety.
But people are sitting outside cafes in surrounding streets and watching the story unfold and lots of people have taken to the streets.
Daniel Sandford : BBC Television Centre : 1450 BST
Scotland Yard have put out their first bulletin.
They've confirmed officers have been deployed at Warren Street in order to carry out an investigation in full protective equipment.
They have also carried out an investigation at the Oval and early indications show there are no traces of chemical agents.
London Transport Police have fantastic new detectors and can get to the scene quickly.
They've clearly got one of these detectors to the Oval very soon after the incident.
This has shown how quickly the police learn lessons.
Two weeks ago witnesses were moved away from the scenes quickly - what the police are trying to do now is to gather people together who might have seen what happened.
These are all crime scenes, and the incidents are serious enough for the police to want to know what happened and who did it.
Rachel Wright : Oval Tube station 1449 BST
The police just asked for all witnesses to come forward and they have been taken off, presumably to give statements.
We've talked to a few eyewitnesses. One talked of the noise of "champagne corks popping" and "acrid smoke".
Another spoke of seeing someone running from the Tube station.
Duncan Walker : Shepherd's Bush : 1443 BST
For the dozens of people milling about by the police cordon, the alert is little more than an inconvenience.
There's no chance of getting the shopping done or travelling into town with the roads closed and the Tube down.
Among those hoping for the all-clear to be given is Reverend Nabil F Shehadi who was asked to be on hand by the Bishop of Kensington.
"People so far have just been asking how they can get home," he said. "The ironic thing is that my next position is as vicar of Beirut and I'm wondering which is safer."
Duncan Walker : Shepherd's Bush : 1441 BST
The normally busy Uxbridge Road has been closed down and all the shops shut.
Hundreds of people have been pushed back away from the station. Among them is Arline Dowling, of Shepherd's Bush, who said: "I'm just really worried that they are telling us all to get back down the road and my daughter is in school, inside the cordon. This doesn't make me feel good at all."
Andrew Marr : Whitehall : 1430 BST
We've spoken to some ministers and they have made clear they know little more than we do.
They will be trying to establish whether this is a series of hoaxes.
The biggest difficulty for the Cobra meeting this afternoon is how you treat these kind of incidents - are we going to see many of them?
And what kind of response is appropriate in such a large city that you want to keep moving?
This may be a relatively short meeting as they are still waiting for hard information.
Originally the prime minister was going to have a press conference with John Howard, but he has been politely ushered out of Downing Street so Tony Blair can turn his attention to these incidents.
Mike Williams : Oval Tube station : 1428 BST
I was talking to a young woman a while ago who said that she was in the carriage, she heard a fuss being made at one end, a young man got off at the station and ran away.
At the same time she said she heard a small detonation and the smell of smoke. I couldn't quite work out from her whether she was talking about just smoke from a fire... she had to leave in a hurry and the police were taking her away.
She seemed to give the impression that it may have been tear gas or something. It would be consistent with a tear gas grenade, a small bang, a flash and the smell of acrid smoke. That was what she seemed to be describing.
The police here say that there have been no casualties as far as they are aware in this particular incident.
Crispin Thorold : Warren Street : 1425 BST
I have spoken to one person who was travelling on the Northern Line. He came off the Northern Line train and saw people coming, running away from the Victoria Line train, which is another line that serves this Tube station.
He said that there was a whole deluge of people sprinting towards him. He said everyone was very panicky and he said quite understandably that as soon as he saw those people he joined them and ran away from whatever had happened.
He did manage to talk to a couple of people. He said someone said something about smoke, someone else said something about a loud bang but he said he really didn't know whether those were things that had actually happened because he said there was so much confusion.
Brian Wheeler : Oval Tube station : 1415 BST
The police cordon has just been moved further back and is now several hundred metres from the underground station. There are 100 or so people gathered by the cordon.
Witnesses to whatever happened are being interviewed by police within the cordon area. There are conflicting accounts of what happened.
One man said he had been told a person had thrown a rucksack on a Tube train and jumped out just before the doors closed, that there was a bang with smoke and people had fled.
He said there had not been injuries as far as he knew. He said a man had been chased as he attempted to run away from the Oval underground station.
James Menendez : Oval Tube station : 1410 BST
I'm around 40 or 50 metres away from the entrance to the Tube and the reason for that is that it has been all sealed off.
There is a major crossroads outside the station and that has all been sealed off, the road has been closed.
The only vehicles that have been allowed in are the 10 or 12 emergency vehicles, all police vehicles as far as I can see, some marked, which are waiting outside but other than that there is no-one else in the station.
That has all been evacuated and that's all the information that they are giving us at the moment.
Peter Hunt : Warren Street : 1407 BST
Well it is an increasingly depressingly familiar scene.
I'm standing in front of a police line. There are two "do not cross" signs in front of me. There is a police officer there. As I look ahead of me I can see Warren Street Tube.
There are four or five fire engines standing there. There is an emergency ambulance standing outside and in the street where there are no ordinary people there are several people in bright orange uniforms.
As I've walked down here a couple of hundred yards I've heard endless police sirens. Once again that sound filling the streets of London.
According to an eyewitness here people have spoken of smelling smoke on the northbound Victoria Line and an eyewitness has spoken of people running off and leaving their shoes and belongings when they smelled that smoke.
We've also seen a sniffer dog in the vicinity. As I watch I can see the sniffer dog walking down one of the roads down an underpass with a police officer.
This whole area is cordoned off, as more and more officers come to this underground station.
Bill Wilson : Shepherd's Bush Tube station : 1405 BST
Two police officers have said there was "an incident on the Tube", with "somebody threatening to blow himself up, but then running off".
Police are everywhere, and a helicopter is hovering above the Tube station.
Traffic is all around, the Tube station is closed and cars have been turned back. There are five or six stationary London buses.
Brian Wheeler : Oval Tube station : 1400 BST
A police officer said that he did not think there had been an explosion on the Northern Line at the Oval.
There had been a separate suggestion there may have been an explosion on the platform.
The atmosphere is calm, the police cordon has been moved back 200 metres from the station.
There are no ambulances, but lots of police vehicles.
Ollie Stone-Lee : Warren Street : 1400 BST
The whole of Tottenham Court Road is now blocked off to vehicles.
Pedestrians can go to within 200 metres of the underground station.
There are large crowds of people outside the many electrical outlets in Tottenham Court Road, following proceedings on television.